Computed tomography (CT), also referred to as computed axial tomography (CAT) or body section roentgenography, is a medical imaging method employing tomography. CT imaging provides diagnostic images of a medical patient for subsequent review by a radiologist or other health care provider. CT images are constructed based on relative radiological densities of the internal organs and tissues obtained from CT scanning. Radiological densities from CT scanning may be expressed in Hounsfield Units (HU). A Hounsfield unit (HU), also referred to as CT number, is a unit used to describe the amount of x-ray attenuation of each “voxel” (volume element) in a three-dimensional image. Voxels are generally represented as 12-bit binary numbers, and therefore have 212=4096 possible values. These values are arranged on a scale from −1024 HU to +3071 HU, calibrated so that −1024 HU is the attenuation produced by air and 0 HU is the attenuation produced by water. Tissue and bone produce attenuations in the positive range.
Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity. The intensity of a pixel is expressed within a given range between a minimum and a maximum, inclusive. This range may be represented as a range from 0 (total absence, black) and 1 (total presence, white), with any fractional values in between. Another convention is to use a percentage scale from 0% to 100%. Grayscale images (e.g., photographs) intended for visual display on monitors or on printed media are commonly stored with 8 bits per pixel. This allows 256 different intensities (i.e., shades of gray) to be recorded, typically on a non-linear scale.
A full density spectrum in CT imaging may produce over 5000 distinct HU density values, and computed images must be able to incorporate over 5000 distinct shades of gray in order to accommodate the full density spectrum. The accuracy provided by 8 bits per pixel format is barely sufficient to display medical images. Uses in medical imaging often require more levels in order to make full use of the sensor accuracy (typically 10, 12 or 16 bits per pixel) and to guard against round-off errors in computations. For example, 16 bits per pixel provides 65,536 levels or shades of gray. File formats, such as a Tagged Image File Format (TIFF), Portable Network Graphics (PNG) and Digital Imaging and Communications in Medicine (DICOM) image file formats, support 16-bit grayscale natively. However, browsers and many imaging programs tend to ignore the low order 8 bits of each pixel.
Although it is preferable to display images with a different shade of gray for every HU density value, the human eye is unable to distinguish between 5000 shades of gray. CT images can be displayed using a process known as windowing. In windowing, a range of HU values are selected for display and all HU values within this range are spread over the available gray scale. For example, high HU values are displayed as white and low HU values as black.
To image an area of the body in which tissues have a similar density—for example, the mediastinum or abdomen—shades of gray can be distributed over a narrow range of HU values (“narrow window”) centered over the average HU value of a particular structure to be evaluated. For example, to evaluate the abdomen in order to find subtle masses in the liver, one might use liver windows. Choosing 70 HU as an average HU value for liver, the shades of gray can be distributed over a narrow window or range. One could use 170 HU as the narrow window, with 85 HU above the 70 HU average value; 85 HU below it. Therefore the liver window would extend from −15 HU to +155 HU, and all shades of gray for the image would be distributed in this range of Hounsfield values. Any HU value below −15 would be pure black, and any HU value above 155 HU would be pure white in this example. Similarly, bone windows would use a “wide window” (to evaluate everything from fat-containing medullary bone that contains the marrow, to the dense cortical bone), and the center or level would be a value in the hundreds of Hounsfield units.
With advances in high digital resolution imaging techniques, parameters for windowing images with 14 bits per pixel (16384 gray levels) are becoming even more important. If the windowing is too narrow or if the optimum level is not selected, image content disappears. If windowing is too wide, the image contrast is too low and details are more difficult to distinguish. Further, inspection and examination of organs and soft tissues may require plurality of viewing windows, each with different upper and lower displayable threshold values. Adjustment of these display windows for the variety of CT images involves considerable user interaction. Since multiple viewing windows must be established for each CT scan slice, radiological diagnoses which involve examination of plurality of internal organs are difficult and time consuming.
Consequently, a need exists for improvements in digital medical imaging techniques. In particular, there remains a need for an improved system and method for displaying a plurality of radiological density images in a single viewing window.